Implantable neuromodulation systems have proven therapeutic in a wide variety of diseases and disorders. Pacemakers and Implantable Cardiac Defibrillators (ICDs) have proven highly effective in the treatment of a number of cardiac conditions (e.g., arrhythmias). Spinal Cord Stimulation (SCS) systems have long been accepted as a therapeutic modality for the treatment of chronic pain syndromes, and the application of tissue stimulation has begun to expand to additional applications such as angina pectoralis and incontinence. Deep Brain Stimulation (DBS) has also been applied therapeutically for well over a decade for the treatment of refractory chronic pain syndromes, and DBS has also recently been applied in additional areas such as movement disorders and epilepsy. Further, in recent investigations, Peripheral Nerve Stimulation (PNS) systems have demonstrated efficacy in the treatment of chronic pain syndromes and incontinence, and a number of additional applications are currently under investigation. Furthermore, Functional Electrical Stimulation (FES) systems, such as the Freehand system by NeuroControl (Cleveland, Ohio), have been applied to restore some functionality to paralyzed extremities in spinal cord injury patients.
These implantable neuromodulation systems typically include one or more electrode carrying modulation leads, which are implanted at the desired stimulation site, and a neuromodulator (e.g., an implantable pulse generator (IPG)) implanted remotely from the stimulation site, but coupled either directly to the modulation lead(s) or indirectly to the modulation lead(s) via a lead extension. The neuromodulation system may further comprise an external control device to remotely instruct the neuromodulator to generate electrical modulation pulses in accordance with selected modulation parameters.
Electrical modulation energy may be delivered from the neuromodulator to the electrodes in the form of a pulsed electrical waveform. Thus, modulation energy may be controllably delivered to the electrodes to stimulate neural tissue. The combination of electrodes used to deliver electrical pulses to the targeted tissue constitutes an electrode combination, with the electrodes capable of being selectively programmed to act as anodes (positive), cathodes (negative), or left off (zero). In other words, an electrode combination represents the polarity being positive, negative, or zero. Other parameters that may be controlled or varied include the amplitude, duration, and rate of the electrical pulses provided through the electrode array. Each electrode combination, along with the electrical pulse parameters, can be referred to as a “modulation parameter set.”
With some neuromodulation systems, and in particular, those with independently controlled current or voltage sources, the distribution of the current to the electrodes (including the case of the neuromodulator, which may act as an electrode) may be varied such that the current is supplied via numerous different electrode configurations. In different configurations, the electrodes may provide current or voltage in different relative percentages of positive and negative current or voltage to create different electrical current distributions (i.e., fractionalized electrode configurations).
As briefly discussed above, an external control device can be used to instruct the neuromodulator to generate electrical modulation pulses in accordance with the selected modulation parameters. Typically, the modulation parameters programmed into the neuromodulator can be adjusted by manipulating controls on the external control device to modify the electrical stimulation provided by the neuromodulator system to the patient. However, the number of electrodes available combined with the ability to generate a variety of complex modulation pulses, presents a vast selection of modulation parameter sets to the clinician or patient.
To facilitate such selection, the clinician generally programs the neuromodulator through a computerized programming system. This programming system can be a self-contained hardware/software system, or can be defined predominantly by software running on a standard personal computer (PC). The PC or custom hardware may actively control the characteristics of the electrical stimulation generated by the neuromodulator to allow the optimum modulation parameters to be determined based on patient feedback or other means and to subsequently program the neuromodulator with the optimum modulation parameter set or sets, which will typically be those that stimulate all of the target tissue in order to provide the therapeutic benefit, yet minimizes the volume of non-target tissue that is stimulated. The computerized programming system may be operated by a clinician attending the patient in several scenarios.
Often, multiple timing channels are used when applying electrical stimulation to target different tissue regions in a patient. For example, in the context of SCS, the patient may simultaneously experience pain in different regions (such as the lower back, left arm, and right leg) that would require the electrical stimulation of different spinal cord tissue regions. In the context of DBS, a multitude of brain structures may need to be electrically stimulated in order to simultaneously treat ailments associated with these brain structures. Each timing channel identifies the combination of electrodes used to deliver electrical pulses to the targeted tissue, as well as the characteristics of the current (pulse amplitude, pulse duration, pulse frequency, etc.) flowing through the electrodes.
The electrical modulation energy may be delivered between electrodes as monophasic electrical energy or multiphasic electrical energy. Monophasic electrical energy includes a series of pulses that are either all positive (anodic) or all negative (cathodic). Multiphasic electrical energy includes a series of pulses that alternate between positive and negative. For example, multiphasic electrical energy may include a series of biphasic pulses, with each biphasic pulse including a cathodic (negative) stimulation phase and an anodic (positive) charge recovery phase that is generated after the stimulation phase to prevent direct current charge transfer through the tissue, thereby avoiding cell trauma and electrode degradation via corrosion. That is, charge is conveyed through the electrode-tissue interface via current at an electrode during a stimulation period (the length of the modulation pulse), and then pulled back off the electrode-tissue interface via an oppositely polarized current at the same electrode during a recharge period (the length of the recharge pulse). Each biphasic pulse has an interphase that defined the time period between the stimulation phase and the charge recovery phase.
In the context of an SCS procedure, one or more leads are introduced through the patient's back into the epidural space, such that the electrodes carried by the leads are arranged in a desired pattern and spacing to create an electrode array. After proper placement of the leads at the target area of the spinal cord, the leads are anchored in place at an exit site to prevent movement of the leads. To facilitate the location of the neuromodulator away from the exit point of the leads, lead extensions are sometimes used. The leads, or the lead extensions, are then connected to the IPG, which can then be operated to generate electrical pulses that are delivered, through the electrodes, to the targeted spinal cord tissue. The modulation, and in the conventional case, the stimulation, creates the sensation known as paresthesia, which can be characterized as an alternative sensation that replaces the pain signals sensed by the patient. The efficacy of SCS is related to the ability to modulate the spinal cord tissue corresponding to evoked paresthesia in the region of the body where the patient experiences pain. Thus, the working clinical paradigm is that achievement of an effective result from SCS depends on the modulation lead or leads being placed in a location (both longitudinal and lateral) relative to the spinal tissue such that the electrical modulation will induce paresthesia located in approximately the same place in the patient's body as the pain (i.e., the target of treatment).
Although alternative or artifactual sensations are usually tolerated relative to the sensation of pain, patients sometimes report these sensations to be uncomfortable, and therefore, they can be considered an adverse side-effect to neuromodulation therapy in some cases. It has been shown that high-frequency pulsed electrical energy can be effective in providing neuromodulation therapy for chronic pain without causing paresthesia. In contrast to conventional neuromodulation therapies, which employ low- to mid-frequencies to efficiently induce desired firing rate of action potentials from electrical pulses (e.g., one pulse can induce a burst of action potentials, or multiple pulses may be temporally integrated to induce on action potential), high frequency modulation (e.g., 1 KHz-50 KHz) can be employed to block or otherwise disrupt naturally occurring action potentials within neural fibers or otherwise disrupt the action potentials within the neural fibers. Although high-frequency modulation therapies have shown good efficacy in early studies, it would be desirable to provide high-frequency modulation therapy.
Once programmed, current neuromodulation systems are designed to deliver tonic modulation pulse trains in each timing channel (i.e., the pulse amplitude, pulse rate, pulse width, and interphase) are fixed. As a result, current neuromodulation systems are only capable of generating a simple pulse train for each of the timing channels. However, in some cases, it may be desirable to generate more complex pulse trains, which may be useful in controlling the response in neurons. For example, neuron response is a dynamic time course that can vary with sequential modulation. It is hypothesized that there is a temporal integration of the modulation effects induced from multiple pulses if the train of pulses is programmed within the responsive time frame of neuron. It would, thus, be desirable to provide therapy using complex pulse trains.
While neuromodulation systems can be designed with hardware capable of generating complex and/or high frequency pulse trains, redesigning the hardware on presently existing neuromodulation designs to accommodate these pulse trains may be a monumental task. Furthermore, neuromodulation systems that are currently used in the field may not be easily updated to generate these pulse trains.
There, thus, remains a need to provide an improved technique for more easily enabling presently existing neuromodulation systems to generate more complex and/or higher frequency pulse trains.